The specificity of C57BL/10 cytotoxic effector cells generated by in vitro sensitization with autologous lymphocytes modified with a series of related nitrophenyl compounds was investigated. Effector cells generated by sensitization with autologous cells modified by the trinitrophenyl (TNP) group did not lyse target cells in which the TNP group was separated from the cell surface by a tripeptide spacer group. The same effectors lysed targets in which the TNP group was coupled directly to the cell surface. These results demonstrate the high degree of fine specificity exhibited by these effector cells, and indicates that there is not a distinct receptor on the cytotoxic T-lymphocyte which recognizes the TNP moiety exclusively. BIBLIOGRAPHIC REFERENCE: Rehn, T. G., Inman, J. K., and Shearer, G. M.: Cell-mediated lympholysis to H-2 matched target cells modified with a series of nitrophenyl compounds. J. Exp. Med. 144: 1134-1140, 1976.